In this paper the low temperature deposition of nanocrystalline and ultrananocrystalline diamond (UNCD) films is compared and discussed. NCD films were prepared by hot filament chemical vapor deposition from a 1% CH4/H2 mixture, while microwave plasma chemical vapour deposition was used to deposit UNCD films from a mixture of 17% CH4/N2. The resulting films have been thoroughly characterized concerning their morphology and structure by scanning electron microscopy and concerning their crystalline properties by X-ray diffraction. The composition was analyzed by X-ray photoelectron spectroscopy (XPS), whereas XPS and Raman spectroscopy were applied to get information on the bonding structure of the films. The most important result of this study is that the composition, structure, morphology, and bonding environment of UNCD hardly change if the deposition temperature is lowered from 770 to 530 8C or even 450 8C. In contrast, there are drastic changes of the nature of NCD films if the temperature is reduced to 700 8C or even lower. Interestingly, the sp2/sp3 ratio of the NCD films remains low and constant in the temperature range investigated. Rather, the nature of the sp2 grain boundary material undergoes drastic changes if the temperature is lowered below 700 8C. In addition, the films become inhomogeneous on a micrometer (not nanometer) scale. Possible reasons for these observations will be discussed throughout the paper.